Positional information providing services providing various terminals with positional information of a PHS (Personal Handyphone System) terminal, mobile phone, or other mobile unit through a communications network are being commercially offered. The positional information providing systems providing these services are connected to mobile communication networks or the Internet etc., acquire positional information of a mobile unit in response to a request from the terminal, and transmit the information to the terminal to be notified.
For example, in a mobile communications network, the ID of the wireless zone in which the mobile unit is located, or the ID of the exchange unit having jurisdiction over the wireless zone is registered in a service control office for each mobile unit stored in the mobile communications network for a packet communication and the like. Current position searching services and position tracking services using these wireless zone IDs and exchange IDs as positional information of the mobile units have already been established.
Further, some of these positional information providing services can provide service users with high precision positional information of the mobile units measured by using GPS (Global Measuring System) receivers when these GPS receivers are built into the mobile units.
Here, when a positional information providing system acquires positional information of a mobile unit with a built-in GPS receiver, the positional information providing system can acquire the positional information of the mobile unit by a total of two types of systems, which are, the system of acquiring the wireless zone ID of the mobile unit or the exchange ID from the service control office of the mobile communications network, and the system of acquiring the positional information of the mobile unit by using the GPS receiver. The mobile units which are able to measure position by using a plurality of measuring systems have been marketed in recent years. Along with the spread of these mobile units, positional information providing systems have been augmented in function so as to be able to provide positional information on mobile units by various measuring systems.
Service users receiving the positional information of a mobile unit by using such a positional information providing service, however, place various demands on the positional information supplied. For example, they want the current positional information, not the one more than 30 minutes ago, want positional information of as high a precision as possible, want to acquire positional information at as low a rate as possible even at the expense of a lower precision, want to acquire positional information in as short a time as possible, etc.